The invention relates to a control device and a control method for a reducing agent injection device, each of which controls a reducing agent injection device, and to a reducing agent injection device that injects a reducing agent into an exhaust passage of an internal combustion engine.
Exhaust gas of an internal combustion engine, such as a diesel engine, mounted on a vehicle includes NOx (nitrogen oxides). As a device that reduces and breaks down such NOx into nitrogen, water vapor, and the like so as to purify the exhaust gas, a urea selective catalystic reduction (SCR) system has been in practical use. The urea SCR system is a system that uses a urea aqueous solution as a reducing agent and makes NOx in the exhaust gas react with ammonia to break down NOx.
Such a urea SCR system includes: a selective reduction catalyst that is disposed in an exhaust passage; and a reducing agent injection device that injects the urea aqueous solution into a portion of the exhaust passage on an upstream side of the selective reduction catalyst. The selective reduction catalyst is a catalyst with a function of: adsorbing ammonia that is produced when the urea aqueous solution is broken down; and promoting a reduction reaction of ammonia with NOx in the inflow exhaust gas. The reducing agent injection device includes: a pump that pressure-feeds the urea aqueous solution accommodated in a storage tank; an injector that injects the urea aqueous solution pressure-fed by the pump; and a control device that controls the pump and the injector.
A freezing point of the urea aqueous solution, which is used in the urea SCR system, differs by concentration. A temperature thereof at the lowest freezing point is approximately 11° C. below zero. Thus, the urea aqueous solution is collected in the storage tank from the system during a stop of the internal combustion engine in order to avoid a situation where the urea aqueous solution is frozen during a stop of the vehicle, and a volume thereof is thereby increased, which causes damage to the pump, the injector, a pipe through which the urea aqueous solution flows, or the like. The collected urea aqueous solution is refilled in the system at a next start-up of the internal combustion engine.
In the urea SCR system, when the remaining urea aqueous solution in the injector is heated by heat that is transmitted from an exhaust pipe during the stop of the internal combustion engine, moisture is possibly evaporated, and urea crystals are possibly precipitated. More specifically, in the case where a temperature of the injector is dropped to an atmospheric temperature after the urea crystals are precipitated due to evaporation of the moisture or after urea concentration in the urea aqueous solution is increased due to the evaporation of the moisture, the urea crystals that can no longer be dissolved in the aqueous solution are possibly precipitated. When such urea crystals are precipitated, a valve body of the injector is stuck, which becomes a cause of actuation failure of the injector at the next start-up of the internal combustion engine.
However, because crystallized urea is likely to be dissolved in water, the actuation failure of the injector is solved by supplying the urea aqueous solution to the injector. In addition, such crystals are melted at a high temperature. Thus, when an exhaust temperature is increased, the actuation failure of the injector is solved by the heat that is transmitted via the exhaust pipe. However, the exhaust gas is also produced in a period from the start-up of the internal combustion engine to time at which the urea crystals are dissolved in the urea aqueous solution or melted by exhaust heat. Thus, it is desired to promptly melt the urea crystals.
Here, although not a technique of melting the crystallized urea, a technique of inducing heat in a coil of the injector by energizing the coil to defrost the frozen urea aqueous solution in the injector has been known. For example, JP-A-2013-221425 discloses a technique of supplying a current in such a degree that the urea aqueous solution is not injected to the coil of the injector, so as to induce the heat in the coil when the temperature of the urea aqueous solution in the storage tank is lower than a predetermined lower limit temperature.
However, the technique described in JP-A-2013-221425 has a purpose of defrosting the frozen urea aqueous solution and thus is not a technique of melting the urea crystals that are precipitated in the injector. Furthermore, in the technique described in JP-A-2013-221425, the coil is energized when the temperature of the urea aqueous solution in the storage tank is lower than the lower limit temperature. Meanwhile, the urea crystals can exist at a temperature at which the urea aqueous solution is in a defrosted state. Thus, the technique described in JP-A-2013-221425 has no intention of melting the urea crystals.
If the technique described in JP-A-2013-221425 is applied to melt the urea crystals, the following problems are concerned. That is, members, such as the exhaust pipe, that are adjacent to the injector are also at a low temperature at the start-up of the internal combustion engine. Thus, in the case where the current is supplied to the coil immediately after the start-up, the heat generated by the coil is transferred to peripheral members. For this reason, the coil is energized for a long time before the urea crystals are melted. Therefore, an amount of electric power consumption by a battery is possibly increased. In addition, when an energization time of the coil is extended, thermal deterioration of the coil possibly occurs.